Millions of B lymphocytes are produced daily in the bone marrow, yet little is known about the regulation of that process or the characteristics of early B cell precursors. The goal of this research is to study the immunobiology of B cell differentiation by examining microenvironmental influences on that process and identifying and purifying intermediate B cell precursors. The model system for these studies will be two different long-term bone marrow cultures. The Dexter LTBMC does not allow the production or maintenance of B cells but maintains an early B cell precursor. The Whitlock-Witte LTBMC allows the long-term production of B lymphocytes. Both LTBMC are dependent on an adherent monolayer of stromal cells which appears to duplicate an in vivo microenvironment. A morphologic and functional analysis of the two LTBMC will be made in order to understand these stromal elements. The different features of the two systems will be exploited in order to induce in vitro B cell differentiation by seeding early B cell precursors from Dexter LTBMC onto a Whitlock-Witte monolayer. The effect of stromal cells on B cell reconstitution following bone marrow transplantation will be tested by injecting B cell precursors with and without purified populations of stromal cells into irradiated CBA/N mice. The second aim of these studies will identify intermediate stages of B cell differentiation by examining the in vivo differentiative potential of Dexter LTBMC cells bearing the 14.8 surface marker. Whether or not a B restricted stem cell exists will be determined by injecting limiting dilution numbers of Dexter cells into severe combined immunodeficient C.B-17 scid mice and examining them for selective repopulati on of B lymphocytes only. Purification of B cell precursors will be performed based on cell surface carbohydrate differences by fracti onating Dexter LTBMC cells with a panel of lectins. The differentiative potential of the cell fractions will be tested by grafting them into immunodeficient mice and purified B cell precursor populations so identified will be further characterized and used as a source of cells with which to develop an in vitro colony assay for B cell precursors. These approaches will contribute to a knowledge of the basic biology of normal B cell differentiation. Such information will be critical in the understanding of pathological B cell development, the result of which is a variety of B cell leukemias.